Method for the reuse of gallium nitride epitaxial substrates

ABSTRACT

A method for the reuse of gallium nitride (GaN) epitaxial substrates uses band-gap-selective photoelectrochemical (PEC) etching to remove one or more epitaxial layers from bulk or free-standing GaN substrates without damaging the substrate, allowing the substrate to be reused for further growth of additional epitaxial layers. The method facilitates a significant cost reduction in device production by permitting the reuse of expensive bulk or free-standing GaN substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 ofcommonly-assigned U.S. Utility patent application Ser. No. 14/493,099,filed on Sep. 22, 2014, by Casey Holder, Daniel Feezell, Steven P.DenBaars, and Shuji Nakamura, entitled “METHOD FOR THE REUSE OF GALLIUMNITRIDE EPITAXIAL SUBSTRATES”, which is a continuation under 35 U.S.C.Section 120 of commonly-assigned U.S. Utility patent application Ser.No. 13/767,739, filed on Feb. 14, 2013, by Casey Holder, Daniel Feezell,Steven P. DenBaars, and Shuji Nakamura, entitled “METHOD FOR THE REUSEOF GALLIUM NITRIDE EPITAXIAL SUBSTRATES”, now U.S. Pat. No. 8,866,149,issued Oct. 21, 2014, which application claims the benefit under 35U.S.C. Section 119(e) of commonly-assigned U.S. Provisional PatentApplication Ser. No. 61/600,301, filed on Feb. 17, 2012, by CaseyHolder, Daniel Feezell, Steven P. DenBaars, and Shuji Nakamura, entitled“METHOD FOR THE REUSE OF GALLIUM NITRIDE EPITAXIAL SUBSTRATES”, both ofwhich applications are incorporated by reference herein.

This application is related to the following applications:

U.S. Provisional Patent Application Ser. No. 61/673,966, filed on Jul.20, 2012, by Casey Holder, Daniel F. Feezell, Steven P. DenBaars, andShuji Nakamura, entitled “STRUCTURE AND METHOD FOR THE FABRICATION OF AGALLIUM NITRIDE VERTICAL CAVITY SURFACE EMITTING LASER”; and

U.S. Provisional Patent Application Ser. No. 61/679,553, filed on Aug.3, 2012, by Casey Holder, Daniel F. Feezell, Steven P. DenBaars, andShuji Nakamura, entitled “STRUCTURE AND METHOD FOR THE FABRICATION OF AGALLIUM NITRIDE VERTICAL CAVITY SURFACE EMITTING LASER”;

both of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the reuse of gallium nitride(GaN) epitaxial substrates.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Traditionally, (Al,In,Ga)N nitride devices have been grownheteroepitaxially on substrates such as sapphire and silicon carbide dueto the lack of available bulk GaN substrates [1]. Non-native substrateshave significant disadvantages, including lattice mismatch to GaN (whichcauses strain and deleterious defects such as threading dislocations)and the inability to grow high-quality GaN crystal films withorientations other than c-plane [1].

Non-c-plane orientations, including non-polar and semipolarorientations, exhibit reduced or eliminated internal electric fields inquantum well devices. This strain-induced piezoelectric polarizationresults in the quantum-confined Stark effect (QCSE), whereby a spatialseparation between electron and hole wavefunctions reduces therecombination efficiency in light-emitting quantum wells [2]. Thereduction or elimination of this field in non-polar and semi-polarcrystal orientations can result in improved device performance [3], [4].However, the lack of native bulk substrates has limited the applicationof these crystal orientations.

Recently, low-dislocation-density bulk GaN substrates have becomecommercially available [1], [5]. This has allowed for the realization ofc-plane and non-c-plane devices with low threading dislocation (TD)densities [5]. It has also eliminated the lattice and thermal expansioncoefficient mismatch problems that come with using non-nativeheteroepitaxy substrates. However, high cost has limited the adoption ofbulk GaN substrates. Work continues towards bringing down the cost ofbulk GaN substrates, a major barrier to widespread adoption of thesepreferential substrates [1], [5].

Substrate removal has previously been used in III-nitrides and othermaterials systems. For instance, GaN-based devices can be removed fromsapphire substrates using laser liftoff. Flip-chip bonding has foundapplication in technologies like processing of LEDs [6] and GaN poweramplifiers [7]. However, it has not been feasible to remove bulk GaNsubstrates from homoepitaxially-grown films or devices. The ability toremove bulk GaN substrates from electronic and optoelectronic devices,without damaging the substrate itself, could allow for the reuse ofthese expensive substrates, substantially reducing the cost ofindividual devices.

SUMMARY OF THE INVENTION

The present invention discloses a device, comprising a III-nitrideepitaxial film grown on a used III-nitride substrate. The epitaxial filmcan comprise an optoelectronic, electronic, or thermoelectric devicestructure. In one or more embodiments, the device structure'sperformance is not degraded as compared to a device structure grown on anew or non-used III-nitride substrate.

In one or more embodiments, the substrate has been used to grow at least20 devices.

The epitaxial film can be grown on an etched surface or etched andpolished surface of the substrate.

The surface of the substrate can be epitaxy ready, with no difference incrystal quality, doping, and surface roughness as compared to a newepitaxy ready substrate.

The epitaxial film can be grown on or above a sacrificial layer grown onor above the substrate.

The substrate can be a bulk or free standing III-nitride substrate, forexample.

One or more embodiments of the present invention use band-gap-selectivephotoelectrochemical (PEC) etching to remove one or more epitaxiallayers from bulk or free-standing GaN substrates without damaging thesubstrate, allowing the substrate to be reused for further growth ofadditional epitaxial layers.

The etch can be a wet etch or a band-gap-selective photoelectrochemical(PEC) etch, for example.

The etch can be performed on a sacrificial etch layer between theepitaxial layers and the substrate.

The substrate can be a polar, semipolar or nonpolar orientation of GaN.

The epitaxial layers can be sub-mounted to a carrier substrate beforethe etch is performed.

The substrate can be an intact and reusable substrate following theetch.

The substrate can be prepared for reuse following the etch byplanarization techniques, such as etching, lapping, polishing, etc.

The substrate can be prepared for reuse following the etch by regrowthtechniques, such as metalorganic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), etc.

The additional epitaxial layers can be grown on the substrate followingthe etch.

One or more embodiments of the technique(s) described here facilitate asignificant cost reduction in device production by permitting the reuseof expensive bulk or free-standing GaN substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIGS. 1(a) and 1(b) are a pair of scanning electron microscope (SEM)images showing epitaxial device layers after removal from a bulk GaNsubstrate, leaving the GaN substrate intact.

FIGS. 2(a), 2(b), 2(c) and 2(d) together illustrate an exemplary processflow for substrate removal using PEC etching.

FIG. 3 illustrates a method of fabricating a device.

FIG. 4(a) illustrates a Vertical Cavity Surface Emitting Laser (VCSEL)device structure.

FIG. 4(b) shows a scanning electron microscope (SEM) image of threecompleted VCSEL devices.

FIG. 4(c) shows the near-field pattern of a 10-μm-diamater deviceoperating above threshold.

FIG. 4(d) illustrates light vs. current (L-I) curve for a nonpolarm-plane GaN VCSEL lasing under pulsed operation at 0.03% duty cycle.

FIG. 4(e) illustrates optical emission spectrum of a nonpolar m-planeGaN VCSEL lasing under pulsed operation at 0.3% duty cycle.

FIG. 4(f)-(g) illustrate a normalized light intensity vs. polarizerangle at various currents above and below threshold for the device shownin FIG. 4(a).

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Technical Description

As noted above, substrates for the homoepitaxial growth of GaN,particularly bulk and non-c-plane substrates, are a significant portionof the cost of GaN devices, including Light Emitting Diodes (LEDs),laser diodes, transistors, and photovoltaic cells. While heteroepitaxysubstrates such as sapphire can be removed using processes such as laserliftoff, no method exists for the removal and subsequent reuse of bulkor free-standing GaN substrates used for homoepitaxy of GaN. This istrue for GaN substrates of polar, semipolar, and nonpolar orientations.

The present invention uses band-gap-selective photoelectrochemical (PEC)etching to nondestructively remove bulk GaN substrates without damagingthe substrate. Due to the bandgap-selective nature ofPhotoelectrochemical (PEC) etching, it is possible to selectively etchepitaxially-grown sacrificial layers, thereby removing devices from thegrowth substrate without damaging the devices or the substrate [8]. Sucha process leaves the entire substrate intact. With an appropriatechemical-mechanical planarization process, the removed substrate couldbe reused again for further epitaxial growth and processing of devices.Since the substrate itself represents a significant portion of the costof devices grown on bulk GaN, the ability to reuse substrates multipletimes could yield substantial cost reductions on a per-device basis.

The removal of a bulk GaN substrate from individual electronic andoptoelectronic devices using photoelectrochemical etching of asacrificial layer) has been demonstrated, showing this process to befeasible.

FIGS. 1(a) and 1(b) are a pair of scanning electron microscope (SEM)images showing epitaxial device layers after removal from a bulk GaNsubstrate, leaving the GaN substrate intact. Specifically, FIG. 1(a)shows epitaxially-grown layers 100 bonded 102 to a submount 104 aftersubstrate removal using PEC etching. FIG. 1(b) shows a bulk GaNsubstrate 106, which is intact with metal contacts, after substrateremoval using PEC etching.

The application of this invention to a product can follow the processflow outlined in FIGS. 2(a), 2(b), 2(c) and 2(d), as one example ofmultiple possible applications. Specifically, FIGS. 2(a), 2(b), 2(c) and2(d) together illustrate an exemplary process flow for substrate removalusing PEC etching.

FIG. 2(a) shows epitaxial growth of an epitaxial layer 200, asacrificial layer 202 (e.g., In_(x)Ga_(1-x)N where 0<x≦1), and one ormore device layers 204, on a bulk GaN substrate 206.

FIG. 2(b) shows device processing including a vertical etch to exposethe sacrificial layer 202.

FIG. 2(c) shows wafer bonding the device layers 204 to carrier wafer orsubmount 208.

FIG. 2(d) shows substrate removal through PEC etching. These steps aredescribed in more detail below.

An epitaxy-ready bulk GaN substrate, of any orientation, is producedinternally or purchased from a commercial vendor. Epitaxial filmsincluding device layers 204 are grown on this substrate 206 to formelectronic, optoelectronic or thermoelectric devices, such as LEDs,laser diodes, photovoltaic cells, transistors, or thermoelectricdevices, as shown in FIG. 2(a). Before (or after) growing any number ofthe device layers 204, a sacrificial In_(x)Ga_(1-x)N layer 202 is grownoverlying the substrate 206. This sacrificial layer 202 could becomprised of one InGaN film of any thickness, or of multipleIn_(x)Ga_(1-x)N films (such as in the form of a superlattice). Thissacrificial layer 202, placed underneath the device 204, can beselectively etched, without substantial etching of the surrounding GaN(including the GaN substrate 206), higher-band-gap InGaN, or AlGaNlayers through the use of band-gap-selective PEC etching.

After epitaxial growth, processing takes place according to therequirements for the desired device. Either as a part of the devicefabrication or as a separate added step, a vertical dry or wet etch isperformed to expose the sacrificial layer 202, as shown in FIG. 2(b).Additionally, a metal contact may be made to the n-type GaN grownunderneath the sacrificial layer, which may aid in the extraction ofcarriers and may be a separate requirement from any contacts made to thedevices themselves.

The devices can then be bonded to a carrier substrate 208 using one ofmany possible methods for flip-chip bonding, such as Au—Au bonding orAuSn eutectic bonding, as shown in FIG. 2(c). The sample can then besubmerged in any appropriate electrolyte solution (including, but notlimited to, KOH, HCl, HNO₃, etc.), and exposed to light that is abovethe bandgap of the In_(x)Ga_(1-x)N sacrificial layer 202, but below thebandgap of GaN. The latter condition is a requirement that allows foretching of the In_(x)Ga_(1-x)N sacrificial layer 202 while preventingundesirable etching of other surrounding layers, such as the GaNsubstrate 206. This light could come from any source, includingnarrow-emission sources like lasers or LEDs, or filtered broadbandsources like a Xe lamp with a long-pass filter (using GaN itself as afilter will allow for any composition of In_(x)Ga_(1-x)N to be etchedwhile not allowing etching of GaN). The sacrificial In_(x)Ga_(1-x)Nlayer 202 will etch laterally, and after some time will be completelyremoved (e.g., undercut), freeing the substrate 206 from the individualdevices that were bonded to a carrier substrate 208, as shown in FIG.2(d). The etch process may or may not include an applied bias ortemperature control, which can assist in the etch process.

After substrate removal, the devices including layers 204 (mounted on acarrier 208) can be further processed according to the desired device.The removed substrate 206 may still have metal contacts or raised mesaareas on the top surface, which are easily removed with acid etches thatdo not damage the substrate or other simple processing steps. There mayalso be previously-grown epitaxial films on the top surface. The etchprocess used to expose the sidewall of the sacrificial layer can alsoleave some features on the surface, generally very small in height (andthis height can easily be controlled by controlling the depth of thetop-down etch process). These epitaxial layers can then be removed bypolishing, chemical-mechanical planarization (CMP), or other processes,and the surface can be treated to prepare it for further epitaxialgrowth. In general, the substrate may be prepared for reuse followingthe etch by planarization techniques, such as etching, lapping,polishing, etc., and/or by regrowth techniques, such as metalorganicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydridevapor phase epitaxy (HVPE), etc.

Through this process, a bulk GaN substrate can be returned to itsoriginal, epitaxy-ready state after every use. As such, a substrate canbe used multiple times, allowing for a significant cost savings withevery subsequent reuse. The limit on the number of reuse events for thesubstrate will be determined by the portion of the substrate thicknessthat must be removed to prepare the substrate for reuse. At some point,the substrate may become too thin for practical reuse. The monetaryamount saved with each reuse will equal the difference between a newbulk GaN substrate from a commercial vendor and the cost of additionalprocessing required for substrate reuse. These additional processingsteps (and their cost) will vary according to the devices beingfabricated. For instance, an additional metal deposition and patterningstep may be necessary, or this step may be incorporated into anotherstep required for device fabrication.

Additional steps that may always be required are the polishing andsurface preparation of the used substrate. Polishing and surfacepreparation is also required of any new substrate as well, so it ismerely the degree of surface preparation required that will vary betweennew and reused substrates.

Process Steps

FIG. 3 illustrates a method of fabricating a device.

Block 300 represents obtaining or preparing a used III-nitride substrate(e.g., a GaN substrate). The step can comprise performing an etch toremove one or more epitaxial layers from a bulk or free-standing GaN orIII-nitride substrate, without damaging the substrate, thereby allowingthe substrate to be reused for further growth of additional epitaxiallayers. The etch can be a wet etch or a band-gap-selective PEC etch, forexample.

The etch can be performed on a sacrificial etch layer between theepitaxial layers and the substrate.

The substrate can be a polar, semipolar or nonpolar orientation of GaN.

The epitaxial layers can be sub-mounted to a carrier substrate beforethe etch is performed.

The substrate can be an intact and reusable substrate following theetch. For example, the etched surface can have a surface roughnesssmoother than 1 nanometer (nm) root mean square (e.g., on a 10micrometer×10 area as measured by Atomic Force Microscopy). Theroughness can be uniform over the entire wafer (e.g., 2 inch wafer).

In one or more embodiments, the surface of the used substrate afterpreparation is epitaxy ready, with no difference in crystal quality,doping, and surface roughness, etc., as compared to a new epitaxy readysubstrate. For example, after PEC etching to remove the devices and thenprocessing the substrate to make it epi-ready (lap/polish, CMP, chemicaltreatments, etc.), the reusable substrate can be the same as a new one.

However, the substrate can be further prepared for reuse following theetch by planarization techniques, such as etching, lapping, polishing,etc. The substrate can be prepared for reuse following the etch byregrowth techniques, such as metalorganic chemical vapor deposition(MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy(HVPE), etc. The preparation or regrowth can include growth ofadditional epitaxial layer(s) 200 (see FIGS. 2(a)-2(d)). The additionalepitaxial layers (e.g., GaN) can be grown on the substrate following theetch.

Block 302 represents growing an epitaxial film comprising a III-nitrideoptoelectronic or electronic device structure, on or above, oroverlying, the used III-nitride substrate. The epitaxial film can begrown on an etched surface or etched and polished surface of thesubstrate, for example. The epitaxial film can comprise a III-nitrideactive region (e.g. InGaN) between a p-type III-nitride layer or region(e.g., GaN) and an n-type III-nitride (e.g., GaN) layer or region.

The epitaxial film can be grown on or above a sacrificial layer grown onor above the substrate, for example.

Block 304 represents the end result, a substrate or device or devicestructure (e.g., III-nitride film comprising an optoelectronic (e.g.,LED, laser diode), electronic (e.g., transistor), solar cell, orthermoelectric device/device structure). The device can comprise aIII-nitride epitaxial film grown on or above or overlying a usedIII-nitride substrate. For example, the substrate 206 in FIG. 2(a) canbe a used substrate (in which case the device layers 202 comprise anepitaxial film grown on or above a used substrate).

In one or more embodiments, the substrate 206 can have been used to growat least 20 devices and/or the device's performance is not degraded ascompared to a device grown on a new or non-used III-nitride substrate206.

Example: Vertical Cavity Surface Emitting Laser (VCSEL)

FIG. 4(a) shows a schematic illustration of the VCSEL device structure.The epitaxial structure was grown on a free-standing m-plane GaNsubstrate (nominally offcut by 1° in the -c-direction) usingatmospheric-pressure metal-organic chemical vapor deposition. The activeregion is aligned with a peak of the optical standing wave and consistsof 5 In_(0.10)Ga_(0.90)N (7 nm) quantum wells with GaN (5 nm) barriersand a 15 nm Mg-doped Al_(0.20)Ga_(0.80)N electron blocking layer. Anadditional region of 3 In_(0.12)Ga_(0.88)N (7 nm) quantum wells with GaN(5 nm) barriers was embedded in the n-GaN beneath the active region(sacrificial layer). This region was laterally undercut usingband-gap-selective PEC etching after flip-chipping to separate thesubstrate from the devices. The placement of a 15 nm Al_(0.30)Ga_(0.70)Nhole-blocking layer 50 nm above the sacrificial region served to definethe cavity length (7.5λ) and to prevent hole transport to the devicesidewalls during the lateral undercut PEC etch [9]. A mesa was definedby etching through the active region (stopping above the sacrificialregion) and a SiN_(x) dielectric was patterned for protection of theactive region sidewalls during PEC etching and to define a currentaperture, which ranges in diameter from 7 to 10 μm. Approximately 50 nm(λ/4-wave) of ITO was deposited via electron cyclotron resonancesputtering (using an AFTEX-7600 ECR Plasma Deposition System by MES AFTYCorporation) and patterned as a p-type ohmic intra-cavity contact andcurrent spreading layer. A metal ring contact was formed around thecurrent aperture before deposition of a λ/8-wave Ta₂O₅ interlayer toalign the high-absorption ITO with the node of the optical standing waveand a 13-period SiO₂/Ta₂O₅ distributed Bragg reflector (DBR). A secondmesa etch was performed to expose the sidewalls of the sacrificialundercut layer and metal was patterned to form a bonding pad and toelectrically connect the metal ring contact to the submount. The samplewas then bonded to a gold-coated sapphire submount and the sacrificialIn_(0.12)Ga_(0.88)N QWs were laterally etched by bandgap-selective PECetching, using KOH and a 405 nm laser light source, until the substratewas removed. Ohmic ring n-contacts were formed on the bonded mesas inalignment with the current apertures and a top-down bandgap-selectivePEC etch was performed, with the Al_(0.30)Ga_(0.70)N acting as ahighly-selective stop-etch layer. Finally, a 10-period SiO₂/Ta₂O₂ DBRwas deposited.

As indicated above, the epitaxial structure includes a lower-bandgapsacrificial region underneath the device at a well-defined location,such that the location of the In_(x)Ga_(1-x)N layer would define thelength of the vertical cavity to be ideally suited for the desired VCSELdevice.

This lower-bandgap material would typically be In_(x)Ga_(1-x)N ofvariable compositions, such that it could be selectively etched by asuitable light source, such as a filtered broadband source or anarrow-emission light source.

The Indium (In) containing layer should be preferred for thelower-bandgap sacrificial layer because it is easier to etch out thesacrificial layer by etching. This sacrificial region could be a singlelayer or a set of several layers, of any thickness. In addition, thisselective etching is more preferred for nonpolar or semipolar VCSELbecause the quantum-confined Stark effect (QCSE) limits the thickness ofsacrificial layers that can be used with polar/c-plane sacrificiallayers. In polar/c-plane devices, the built-in electric field that isperpendicular to the sacrificial layer separates the electrons and holesto opposite sides of the layer. Since it is holes that participate inPEC etching, this causes non-uniform etching in polar/c-planesacrificial layers, so layer thicknesses must be kept very thin tocompensate. Thus, superlattices must be used and the etching rate maysuffer due to the thin sacrificial layers used: lower surface area andincreased aspect ratio during lateral etching can both limit etchingrate. Nonpolar and semipolar planes limit the QCSE in the sacrificiallayers (remove it completely in the case of nonpolar), and therebyremove the design restrictions placed on the sacrificial layer by QCSEin the polar/c-plane devices.

FIG. 4(b) shows a scanning electron microscope (SEM) image of threecompleted VCSEL devices. FIG. 4(c) shows the near-field pattern of a10-μm-diamater VCSEL device operating above threshold. FIG. 4(d)illustrates light vs. current (L-I) curve for a nonpolar m-plane GaNVCSEL lasing under pulsed operation at 0.03% duty cycle. FIG. 4(e)illustrates optical emission spectrum of a nonpolar m-plane GaN VCSELlasing under pulsed operation at 0.3% duty cycle. FIGS. 4(f) and (g)illustrates a normalized light intensity vs. polarizer angle at variouscurrents above and below threshold for the device shown in FIG. 4(a).

Further information on fabrication of the VCSEL can be found in [10] andU.S. Provisional Patent Application Ser. No. 61/673,966, filed on Jul.20, 2012, by Casey Holder, Daniel F. Feezell, Steven P. DenBaars, andShuji Nakamura, entitled “STRUCTURE AND METHOD FOR THE FABRICATION OF AGALLIUM NITRIDE VERTICAL CAVITY SURFACE EMITTING LASER”, whichapplication is incorporated by reference herein.

Benefits and Advantages

Bulk or free-standing GaN substrates (including polar, semipolar, andnonpolar orientaions) offer significant performance advantages forelectronic and optoelectronic devices over conventional heteroepitaxysubstrates, such as sapphire or silicon carbide, due to the resultingreduction in threading dislocation densities. However, bulk GaNsubstrates represent a significant cost increase over conventionalheteroepitaxy substrates. This invention will allow for the reuse ofbulk GaN substrates, thereby reducing the per-device cost ofoptoelectronic or electronic devices grown on bulk or free-standing GaNsubstrates.

One or more embodiments of the invention obtain high-performanceelectronic and optoelectronic devices produced at a lower cost throughthe reuse of bulk or free-standing GaN substrates, with no degradationof the device performance caused by the reuse of the substrate.

For example, one or more embodiments of the invention can be used tosignificantly reduce the cost of flip-chip bonded electronic,optoelectronic or thermoelectric devices (such as LEDs, laser diodes,photovoltaic cells, transistors, and Photoelectrochemical (PEC) cellsfor gas generation) grown homoepitaxially on reusable bulk orfree-standing GaN substrates. This cost savings would be realizedthrough the option of reusing expensive bulk or free-standing GaNsubstrates of any orientation (c-plane, non-polar, or semi-polar).

Free-standing bulk GaN substrates used for the growth and fabrication ofa variety of optoelectronic or electronic devices can be removed andreused for future growths of similar devices with no degradation in thequality of the substrates or the devices grown on them.

Possible Modifications

The present invention has experimentally demonstrated the bonding ofindividual (In,Al,Ga)N devices to a submount, and has demonstrated theremoval of a free-standing GaN epitaxial substrate throughband-gap-selective (PEC) etching of a sacrificial layer. However, othermethods of removing or etching the substrate are possible.

The present invention has demonstrated the reuse of a bulk GaN substrate(after bonding and removal of individual devices) for subsequentepitaxial growth, as well as device production. However, other reuse ofother substrates or other III-nitride substrates is also included.

Nomenclature

The terms “(AlInGaN)” “(In,Al)GaN”, or “GaN” as used herein (as well asthe terms “III-nitride,” “Group-III nitride”, or “nitride,” usedgenerally) refer to any alloy composition of the (Ga,Al,In,B)Nsemiconductors having the formula Ga_(w)Al_(x)In_(y)B_(z)N where 0≦w≦1,0≦x≦1, 0≦y≦1, 0≦z≦1, and w+x+y+z=1. These terms are intended to bebroadly construed to include respective nitrides of the single species,Ga, Al, In and B, as well as binary, ternary and quaternary compositionsof such Group III metal species. Accordingly, it will be appreciatedthat the discussion of the invention hereinafter in reference to GaN andInGaN materials is applicable to the formation of various other(Ga,Al,In,B)N material species. Further, (Ga,Al,In,B)N materials withinthe scope of the invention may further include minor quantities ofdopants and/or other impurity or inclusional materials.

One approach to decreasing polarization effects in III-nitride devicesis to grow the devices on a nonpolar substrate or nonpolar planes of thecrystal. These include the {11-20} planes, known collectively asa-planes, and the {10-10} planes, known collectively as en-planes. Suchplanes contain equal numbers of gallium and nitrogen atoms per plane andare charge-neutral. Subsequent nonpolar layers are equivalent to oneanother, so the bulk crystal will not be polarized along the growthdirection.

Another approach to reducing polarization effects in III-nitride devicesis to grow the devices on a semipolar substrate or semipolar planes ofthe crystal. The term “semipolar plane” can be used to refer to anyplane that cannot be classified as c-plane, a-plane, or m-plane. Incrystallographic terms, a semipolar plane would be any plane that has atleast two nonzero h, i, or k Miller indices and a nonzero 1 Millerindex. Subsequent semipolar layers are equivalent to one another, so thebulk crystal will have reduced polarization along the growth direction.

REFERENCES

The following references are incorporated by reference herein:

-   [1] R. Dwiliński et al., “Bulk ammonothermal GaN,” Journal of    Crystal Growth, vol. 311, no. 10, pp. 3015-3018, May 2009.-   [2] K. M. Kelchner et al., “Nonpolar AlGaN-Cladding-Free Blue Laser    Diodes with InGaN Waveguiding,” Applied Physics Express, vol. 2, no.    7, p.-, 2009.-   [3] A. Tyagi et al., “AlGaN-Cladding Free Green Semipolar GaN Based    Laser Diode with a Lasing Wavelength of 506.4 nm,” Applied Physics    Express, vol. 3, no. 1, p. 011002, January 2010.-   [4] J. W. Raring et al., “High-Efficiency Blue and    True-Green-Emitting Laser Diodes Based on Non-c-Plane Oriented GaN    Substrates,” Applied Physics Express, vol. 3, no. 11, pp.    112101-112101, 2010.-   [5] K. Fujito, S. Kubo, H. Nagaoka, T. Mochizuki, H. Namita, and S.    Nagao, “Bulk GaN crystals grown by HVPE,” Journal of Crystal Growth,    vol. 311, no. 10, pp. 3011-3014, May 2009.-   [6] O. B. Shchekin et al., “High performance thin-film flip-chip    InGaN—GaN light-emitting diodes,” Applied Physics Letters, vol. 89,    no. 7, p. 071109, 2006.-   [7] J. J. Xu et al., “1-8-GHz GaN-based power amplifier using    flip-chip bonding,” Microwave and Guided Wave Letters, IEEE, vol. 9,    no. 7, pp. 277-279, 1999.-   [8] A. C. Tamboli, M. C. Schmidt, A. Hirai, S. P. DenBaars,    and E. L. Hu, “Photoelectrochemical Undercut Etching of m-Plane GaN    for Microdisk Applications,” Journal of The Electrochemical Society,    vol. 156, no. 10, p. H767, 2009.-   [9] A. Tamboli, M. Schmidt, S. Rajan, J. Speck, U. Mishra, S.    DenBaars, and E. Hu: J. Elecrochem. Soc. 156 (2009) H47.-   [10] C. Holder et. al., “Demonstration of Nonpolar GaN-based    Vertical Cavity Surface Emitting Lasers,” Appl. Phys. Express    5 (2012) 092104.

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method for reusing a III-nitride substrate, comprising: providing an epitaxy-ready bulk III-nitride substrate that has been previously used to grow a III-nitride optoelectronic or electronic device structure; growing one or more InGaN sacrificial layers on or above the III-nitride substrate; growing one or more III-nitride device layers on or above the InGaN sacrificial layers; and selectively etching the sacrificial layers to separate the III-nitride device layers from the III-nitride substrate without damaging the III-nitride device layers or the III-nitride substrate, such that performance of the III-nitride device layers is not degraded and the III-nitride substrate can be reused.
 2. The method of claim 1, further comprising growing an epitaxial layer on or above the III-nitride substrate, wherein the InGaN sacrificial layers are grown on or above the epitaxial layer.
 3. The method of claim 1, wherein the InGaN sacrificial layers are selectively etched using band-gap-selective photoelectrochemical etching.
 4. The method of claim 3, wherein the band-gap-selective photoelectrochemical etching comprises: submerging a sample comprised of the III-nitride substrate, InGaN sacrificial layers and III-nitride device layers in an electrolyte solution; exposing the submerged sample to light that is above the bandgap of the InGaN sacrificial layers, but below the bandgap of the III-nitride substrate or III-nitride device layers, to allow for the etching of the InGaN sacrificial layers, while preventing undesirable etching of the III-nitride substrate or III-nitride device layers.
 5. The method of claim 4, wherein the band-gap-selective photoelectrochemical etching further comprises making contact to an epitaxial layer grown underneath the InGaN sacrificial layers in order to aid in extraction of carriers during the band-gap-selective photoelectrochemical etching.
 6. The method of claim 1, wherein the InGaN sacrificial layers are selectively etched laterally and, after some time, are completely removed, freeing the III-nitride substrate from the III-nitride device layers.
 7. The method of claim 1, wherein an etch is performed to expose the InGaN sacrificial layers before the InGaN sacrificial layers are selectively etched.
 8. The method of claim 1, wherein the selective etching is performed without substantially etching the III-nitride substrate, thereby allowing the III-nitride substrate to be reused for further growth of additional epitaxial layers.
 9. The method of claim 1, wherein the III-nitride device layers are sub-mounted to a carrier substrate before the etch is performed.
 10. The method of claim 1, wherein the performance of the III-nitride device layers is not degraded after the III-nitride device layers are separated from the III-nitride substrate as compared to III-nitride device layers grown on a new or unused III-nitride substrate.
 11. The method of claim 1, wherein the III-nitride substrate is intact and reusable after the III-nitride device layers are separated from the III-nitride substrate.
 12. The method of claim 1, wherein the III-nitride substrate is processed after the III-nitride device layers are separated from the III-nitride substrate to prepare the III-nitride substrate for reuse.
 13. The method of claim 12, wherein the III-nitride substrate is prepared for reuse by planarization techniques.
 14. The method of claim 12, wherein the III-nitride substrate is prepared for reuse by regrowth techniques.
 15. A device or substrate processed using the method of claim
 1. 16. A structure, comprising: an epitaxy-ready bulk III-nitride substrate that has been previously used to grow a III-nitride optoelectronic or electronic device structure; one or more InGaN sacrificial layers on or above the III-nitride substrate; and one or more III-nitride device layers on or above the InGaN sacrificial layers; wherein the InGaN sacrificial layers are selectively etched layers used to separate the III-nitride device layers from the III-nitride substrate without the III-nitride device layers or the III-nitride substrate being damaged, such that performance of the III-nitride device layers is not degraded and the III-nitride substrate can be reused. 